Thin-film transistors (also known as “organic TFT”) utilizing an organic semiconductor in an active layer (hereinafter referred to as “organic semiconductor layer”) may be mentioned as typical elements for constituting organic semiconductor devices.
In the thin-film transistors, the organic semiconductor layer is formed by forming a film of a molecular crystal typified by pentacene in vacuo. Regarding the formation of the organic semiconductor layer by the film formation in vacuo, there is a report that the optimization of the film formation conditions can realize the formation of an organic semiconductor layer having a high level of charge mobility exceeding 1 cm2/V·s (Y. -Y. Lin, D. J. Gundlach, S. Nelson, and T. N. Jackson, “Stacked Pentacene Layer Organic Thin-Film Transistors with Improved Characteristics,” IEEE Electron Device Lett. 18, 606 (1997)). The organic semiconductor layer formed by the film formation in vacuo, however, is generally in a polycrystal form composed of aggregates of fine crystals. Therefore, many grain boundaries are likely to exist, and, in addition, defects are likely to occur. The grain boundaries and defects inhibit the transfer of charges. For this reason, in the formation of the organic semiconductor layer by film formation in vacuo, the organic semiconductor layer as the element for constituting the organic semiconductor device could not have been substantially continuously produced in a satisfactory wide area with homogeneous properties and without difficulties.
On the other hand, a discotic liquid crystal is known as a material having a high level of charge mobility (D. Adam, F. Closss, T. Frey, D. Funhoff, D. Haarer, H. Ringsdorf, P. Schunaher, and K. Siemensmyer, Phys. Rev. Lett., 70, 457 (1993)). In this discotic liquid crystal, however, charge transfer is carried out based on a one-dimensional charge transfer mechanism along the columnar molecular alignment. Therefore, close control of the molecular alignment is required, and this disadvantageously makes it difficult to utilize the discotic liquid crystal on a commercial scale. Any example of success in thin-film transistors using the discotic liquid crystal as a material for constituting the organic semiconductor layer has not been reported yet.
A high level of charge mobility in a liquid crystal state of rodlike liquid crystalline materials such as phenylbenzothiazole derivatives has already been reported (M. Funahashi and J. Hanna, Jpn. J. Appl. Phys., 35, L703–L705 (1996)). Up to now, however, there is no report on any example of success of thin-film transistors utilizing the rodlike liquid crystalline material in the organic semiconductor layer. The rodlike liquid crystalline material exhibits a few types of liquid crystal states, and the charge mobility is likely to increase with enhancing the regularity of the structure of the liquid crystalline material. The transition of the liquid crystalline material to a crystal state having a higher level of regularity of structure, however, results in lowered or no charge mobility. In this case, of course, no properties required of the thin-film transistor are developed.
When a molecular dispersion polymeric material is used as an organic semiconductor material, an organic semiconductor layer, which has uniform charge transfer characteristics over a large area can be formed by coating this organic semiconductor material. The organic semiconductor layer thus formed, however, has a low charge mobility of 10−5 to 10−6 cm2/V·s, and, disadvantageously, the charge mobility depends upon temperatures and electric fields.
The present invention has been made with a view to solving the above problems of the prior art, and an object of the present invention is to provide an organic semiconductor structure comprising an organic semiconductor layer having a relatively large area and uniform and high level of charge transfer characteristics, which have hitherto been regarded as unattainable, a process for producing the same, and an organic semiconductor device.